Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.100.

Allah, the Exalted, ordered that He should be remembered a lot. Describing the wise men and women who ponder His signs, the Qur’an mentions, “those who remember Allah standing, sitting and on their sides,”  Qur’an 3.191 and “those men and women who engage much in Allah’s praise. For them has Allah prepared forgiveness and a great reward.”  Qur’an 33.35 Mujahid explained, “A person cannot be one of ‘those men and women who remember Allah much’ as mentioned in the above verse of the Qur’an, unless he or she remembers Allah at all times, standing, sitting, or lying in bed.”

When asked how much zhikr one should do to be considered as one of “those who remember Allah much,” Ibn Al-Salah said that “much” is “when one is constant in supplicating, in the morning and evening and in other parts of the day and the night as reported from the Prophet, peace be upon him.” Concerning the above Qur’anic verses ‘Ali b. Abi Talha relates that Ibn ‘Abbas said, “All obligations imposed upon man by Allah are clearly marked and one is exempted from them in the presence of a genuine cause. The only exception is the obligation of zhikr. Allah has set no specific limits for it, and under no circumstances is one allowed to be negligent of it. We are commanded to ‘remember Allah standing, sitting and reclining on your sides,’ in the morning, during the day, at sea or on land, on journey or at home, in poverty and in prosperity, in sickness or in health, openly and secretly, and, in fact, at all times throughout one’s life and in all circumstances.”